F1. Field of the Invention
The invention relates to a process and a device making it possible to control the heating of a heated glass sheet in a horizontal furnace for heat treatment and/or bending.
2. Background of the Prior Art
It is well known to use horizontal furnaces for the heating of glass sheets, called tunnels, passed through by sheets conveyed on a bed of rollers. In these furnaces, the glass sheet is gradually heated and exhibits at the output of the furnace a temperature, for example, greater than the softening point of the glass so that the sheet is then sufficiently plastic to be bent and/or tempered.
Such horizontal furnaces comprise a bottom and a crown of refractory products and the addition of heat is obtained by means of resistors, mounted symmetrically on both sides of the roller conveyor. These resistors are distributed in various longitudinal zones, whose number varies with the length of the furnance, the longitudinal zones themselves divided into crosswise zones of equal widths and lengths. By way of illustration, a horizontal furnace with a length of twelve meters and 1.20 meters wide can thus be divided into five longitudinal zones 2.40 meters in length, each of these zones itself being divided into 3 zones 0.40 meter wide. These dimensions are suitable, for example, for a furnace for heating automobile glazing pieces such as side windows.
Even if in the example cited by way of illustration, 15 resistor zones capable of furnishing 15 different powers of heat are available, it is found that a nonuniform heating of the glass sheets occurs. Consequently, it is very difficult to control this differentiated heating so as to heat, for example, more intensely the zones of the glass that must undergo the greatest deformation.
This nonuniformity of the heating of the glass, precisely, this inabilty to regulate it is explained by various reasons. In the first place, it is explained by the radiation of the refractories used for the construction of the bottom and of the crown of the furnace, refractories which re-emit the received heat to the point that, in certain installations, scarcely 45% of the heat received by the glass comes directly from the resistors, the prime source of heat. This radiation of the refractories is performed in all directions in space and results in a leveling of the heat emissions which works against the desired effect of the differentiation of the heating zones. In addition, certain zones of the glazing are systematically less heated than others; actually, between the resistor zones are zones which do not have resistors and which determine crosswise zones and longitudinal zones which do not coincide with the direct heating of the glass. Since the crosswise zones are all the same width, there is a continuous belt parallel to the direction of advance of the glass, which does not contribute directly to the heating of the glass. These shadow zones thus create a differentiated heating that cannot be controlled which is found in the sequence of operations with, for example, a zone of least tempering.
Finally, if by a very thorough regulation, the various phenomena--which, as we have indicated, have opposite effects--are balanced, the furnace all the same is unsatisfactory for certain types of glazings. Let us take the furnace previously given as an example and imaging processing square glazings that are 50 cm on a side. To be received correctly by the processing units downstream from the furnace, the glazing must necessarily be centered which means that systematically these edges will circulate in shadow zones and will not be able to be overheated. In short, to attenuate certain harmful effects due to the difficulty of maintaining the heating, the position of the glass sheet can be acted upon, but there are then created problems of receiving said glass sheets at the output of the furnace.